Research On Dielectric And Energy Storage Performance Of Na_0.5Bi_0.5TiO₃-CaTiO₃ Based Lead-Free Ceramics

The development of new and reliable renewable energy has become the primary challenge of today’s modern society in order to alleviate the energy crisis and the ever-increasing energy demand.Among energy storage components,dielectric capacitors are widely used in pulse power systems because of their miniaturization and integration characteristics.However,the low energy storage density of dielectric capacitors limits its further industrial application.Therefore,improving the energy storage performance of dielectric capacitors has become the current research focus in this field.Among various dielectric materials,Na_0.5Bi_0.5TiO₃ （NBT）has been widely studied for its good ferroelectricity and rich phase transition,while CaTiO₃（CT）is a linear dielectric with high breakdown field strength and low loss.In this paper,NBT is selected as the matrix material,and the phase structure is adjusted by doping with different content of CT to increase the breakdown field strength,and then improve the energy storage density.（1-x）NBT-x CT（0≤x≤40 mol%）ceramics are prepared by the traditional solid-phase synthesis process,with CT and NBT fully solid-dissolved and a single perovskite structure for all components.The phase structure progressively transforms from the rhombohedral phase to the tetragonal phase as CT content increases.It completely transforms into the tetragonal phase when x≥15 mol%.The existence of polar nano-domains（PNRs）makes the dielectric thermogram gradually show obvious frequency dispersion and dispersion phase transition.When x=15 mol%,a strong relaxation characteristic appears,and the relaxation coefficientγ=1.86.The dielectric peak is located near room temperature,which indicates that the component has good dielectric properties at room temperature.The shape of the P-E hysteresis loop gradually changes from an obvious ferroelectric shape to a similar linear shape.When x=20 mol%,the electric hysteresis loop presents a straight line with almost no energy loss.The multi-phase coexistence at x=10 mol%causes the splitting of the current peak.With the further increase of CT content,and current curve changes from a ferroelectric state to a multiphase coexisting state,then to a relaxed state,and finally to a paraelectric state.The 0.85NBT-0.15CT component was selected for further doping with ions of different valence states（hereinafter referred to as NBCT）to explore the influence of B-site doping(Mg²⁺,Zn²⁺,Sn⁴⁺,Ce⁴⁺)and A-site doping(Dy³⁺,La³⁺)on the dielectric and energy storage performance of NBCT.The substitution of Mg²⁺and Zn²⁺for Ti⁴⁺at the B site is beneficial to increase the breakdown field strength of NBCT ceramics.The NBCT-0.5 mol%Mg and NBCT-1 mol%Zn components obtained 2.75 J/cm³and2.95 J/cm³ energy storage density at 210 kV/cm and 260 kV/cm,respectively.Simultaneously,Ce⁴⁺and Sn⁴⁺also enter the B-site doping.Ce⁴⁺plays a significant role in grain refinement and significantly improves breakdown field strength,and Ce⁴⁺has a role in broadening the dielectric peak to improve dielectric stability.Among them,the component NBCT-2 mol%Ce has an energy storage density of 2.53J/cm³ at 230 kV/cm,and maintains excellent temperature stability in the temperature range of 30-150℃.Sn⁴⁺doping increases the sintering temperature of ceramics,increases the average size and density of crystal grains,and improves breakdown field strength.The energy storage density of the component NBCT-1 mol%Sn reaches 2.34J/cm³ at 220 kV/cm.Both Dy³⁺and La³⁺enter the A site and act as a pinning agent to inhibit grain growth.But in fact,Dy³⁺has a more uniform grain size than La³⁺doping.NBCT-0.5 mol%Dy obtains an energy storage density of 2.35 J/cm³ at 210 kV/cm.Compared with La³⁺doped NBCT,the energy storage density has been improved.In addition,the A-site doped NBCT-0.5 mol%Dy and B-site doped NBCT-1 mol%Sn components were selected for film rolling preparation,and the influence density on energy storage performance was explored.The NBCT-0.5 mol%Dy rolled film sample obtained an energy storage density of 6.02 J/cm³ at 440 kV/cm,and the NBCT-1mol%Sn rolled film sample obtained an ultra-high energy storage density 6.98 J/cm³at 460 kV/cm.Both of them have excellent performance in variable temperature,frequency conversion and charge and discharge test performance.In this paper,on the basis of NBT,the phase structure is adjusted by doping linear dielectric CT,and a novel relaxor ferroelectric ceramic is obtained at room temperature.Furthermore,the A/B-site ion doping can effectively improve the energy storage performance.Moreover,the B-site doping has a more obvious improvement effect on the energy storage performance,and the A-site doping has a significant effect on crystal fineness,which can provide a reference for the regulation of the microstructure of many dielectric ceramics.In addition,the improvement of the preparation process can help to obtain high-density ceramics,improve the electric field tolerance of the ceramics,and thus significantly improve the energy storage performance,which also brings certain guiding significance to the preparation of dielectric ceramic materials for capacitors.